Radiopharmaceutical chelate systems have been used to couple with peptides and proteins, for the purpose of diagnostic imaging. Many such chelate systems have a central "N.sub.2 S.sub.2 ", "N.sub.3 S.sub.3 " moiety; a few have a "N.sub.3 S.sub.2 " moiety.
The N.sub.3 S.sub.2 containing system is preferred due to the combination of one central alkyl amino group and two other alkyl/amide nitrogens and two thiols which are responsible for the chelating properties, and which form stable compounds with Group VIIb metals at reasonable temperatures (i.e., .ltoreq.60.degree. C.,) and which also allow simple attachment to peptides and proteins under aqueous conditions.